A tiny unborn hummingbird is getting in the way of a big bridge project in the San Francisco Bay Area.

The discovery of a nest and egg in a tree is stalling the start of upgrades on the Richmond-San Rafael Bridge about 30 miles north of San Francisco, officials said Tuesday.

The species, Anna's Hummingbird, is protected under the Migratory Bird Treaty Act that forbids the removal of the egg and offers other protections to birds.

The nest - about half the size of a fist - was discovered about a week ago when work was set to begin.

It was found on the Richmond side of the $70 million bridge project, in one of about two dozen trees that were to be removed to widen the freeway, officials said.

Under the protection act, the tree must stay put until the hummingbird baby is gone.

The goals of the act are to protect, restore and manage migratory bird populations to ensure long-term sustainability.

"We've dealt with this on all sorts of things on every project we've worked with in the Bay Area," said Metropolitan Transportation Commission spokesman Randy Rentschler.

Audubon California spokesman Garrison Frost said cliff swallows were building their intricate nests beneath an overpass in Petaluma, and cormorants held up deconstruction of the old Oakland-San Francisco Bay Bridge.

Some work on the Richmond-San Rafael Bridge could be delayed a few weeks by the discovery of the egg.

But Rentschler said the lag is not expected to increase the cost of the project because there are other tasks that crews can do in the meantime.

The project includes adding a bike path on the upper deck of the bridge that will allow riders to cross in both directions. Plans also call for a third traffic lane on the lower deck to ease congestion for drivers heading eastbound.